The Doctor and Clara travel to Caliburn House to investigate the repeated sightings of a mysterious ghost, The Witch in the Well. However, the Doctor soon discovers that the phenomena is anything but supernatural, but a rescue attempt traps him in a collapsing pocket dimension.

Recap

At Caliburn House in 1974 England, it's a dark and stormy night. Inside, psychic Emma Grayling checks with Alec Palmer, who is setting up monitoring equipment. He dismisses the interference as stray FM broadcasts and assures her that he's taken care of it. Alec asks if Emma wants to go through with it again, and she says that "she" is so lonely. Satisfied, Alec begins an audio journal and Emma calls out to the spirit of the Witch of the Well, trying to establish contact. As she offers to bring it home, Alec's equipment overloads and a ghostly figure appears briefly. Emma collapses from the psychic impressions and says that "she" is dead. ..

Clara: Say we actually find her. What do we say?The Doctor: We ask her how she came to be. Whatever she is?Clara: Why?The Doctor: Because I don't know. And ignorance is... umm, what's the opposite of bliss?Clara: Carisle?The Doctor: Yes. Yes, Carisle. Ignorance is Carisle.

The Doctor: Do you feel anything?Clara: No.The Doctor: Your pants are so on fire.

The Doctor: What do you think?Clara: Uh, color's a bit boisterous.The Doctor: I think it brings out my eyes.Clara: Makes my eyes hurt.

Clara: I mean, one moment you're in 1974 looking for ghosts. But all you have to do is open your eyes and talk to whoever is standing there. To you I haven't been born yet. And to you I've been dead a hundred billion years. Is my body out there somewhere, in the ground?The Doctor: Yes, I suppose it is.Clara: But here we are, talking. So I am a ghost. To you, I'm a ghost. We're all ghosts to you. We must be nothing.The Doctor: No, no. You're not that.Clara: Then what are we? What can we possibly be?The Doctor: You are the only mystery worth solving.

The Doctor: Listen, all I need to do is dive into another dimension, find the time traveler, help her escape the monster, get home before the entire dimension collapses, and Bob's your uncle.

Cultural References

Clara: Ghostbusters.

Ghostbusters was a 1984 comedy about four men who exorcise ghosts and call themselves "The Ghostbusters."

The Doctor: Birds do it, bees do it, even educated fleas do it.

The Doctor quotes from the 1928 Cole Porter song, "Let's Do It, Let's Fall in Love."